1. Technical Field of the Invention
The present invention relates to compositions which can be crosslinked by the cationic route to provide hard scratch-resistant coatings.
More preferably, the present invention relates to compositions which can be crosslinked by the cationic route to provide hard coatings furthermore exhibiting anti-fouling and/or anti-mist properties.
2. Description of Background and/or Related and/or Prior Art
Thermoplastics, such as polycarbonate, have acquired a predominant role in numerous applications as substituents for glass. This is the case, for example, in the automobile field, where they are used for the manufacture of lenses for the headlamp units and tail lights of vehicles. This is also the case in the field of the spectacle trade, where they are used for the manufacture of spectacle glasses. The main advantage of these thermoplastics is that they are lighter and less brittle than glass.
However, these materials also exhibit a major disadvantage, namely, their low hardness in comparison with that of glass. Consequently, these materials are more easily subject to scratching and to detrimental changes, even in the context of normal use.
Need thus continues to exist for solutions to limit these problems of scratching and of detrimental change.
One of the solutions employed consists in producing a hard coating at the surface of the thermoplastic, in the form of a transparent laminate intended to improve the performance of the thermoplastic. Numerous documents of the state of the art disclose compositions intended to form this type of coating.
Compositions based on epoxyalkoxysilanes which can be crosslinked by the thermal route are described in U.S. Pat. No. 4,211,823. These compositions make it possible to obtain hard polysiloxane coatings.
A significant disadvantage of these compositions is the time necessary for the crosslinking.
WO-A-94/10230 provides an improvement for avoiding excessively long crosslinking times. This improvement consists in using ultraviolet radiation as means for activating the crosslinking. However, a disadvantage of these compositions is that of having a poorer resistance to abrasion than that of the coatings crosslinked by the thermal route.
In order to overcome this disadvantage, WO-A-02/00561 claims a process for the manufacture of a coating obtained by crosslinking, by the cationic route, an epoxy monomer essentially based on glycidyl, first by photopolymerization and then by thermal post-crosslinking in the presence of a thermal crosslinking catalyst.
U.S. Pat. No. 6,210,790 claims a colloidal silica modified by epoxy or propenyl ether groups and introduced into compositions which can be crosslinked by the cationic route to give a hard coating. The colloidal silica is grafted with the alkoxysilane in water. In addition to the silica functionalized by the alkoxysilane, the composition comprises multifunctional monomers and in particular siloxane monomers with epoxy units and a cationic initiator of onium salt type.
One disadvantage of the technique described in this patent is, first, that it involves the preliminary functionalization of the colloidal silica by an alkoxysilane and, secondly, that it is necessary to remove the solvent for functionalization of the silica before carrying out the crosslinking.
Furthermore, the industries in the technical fields under consideration remain vigilant for hard coatings also having anti-mist and/or anti-fouling properties.
Some prior documents disclose means which make it possible to obtain anti-mist or anti-fouling properties have already been described.
This is the case in particular with WO-A-02/12404, which describes a hard coating based on colloidal silicas which are functionalized by acrylates for optical devices exhibiting anti-fouling properties via a layer of perfluoropolyether deposited on the hard coating.
FR-A-2,749,587 describes a composition which can be crosslinked by radiation to give a hard coating exhibiting anti-mist properties. This composition comprises a colloidal silica, an olefin comprising at least two sites of unsaturation and at least one divalent oxyalkylene radical, and a trialkoxysilane comprising an olefinic functional group.
The anti-mist coating obtained from the composition described in FR-A-2,749,587 has the principal disadvantage of being sensitive to inhibition by atmospheric oxygen.
However, no composition of the prior art is capable of providing a hard coating jointly exhibiting anti-mist and anti-fouling properties. This is because the mechanisms of the anti-mist and anti-fouling properties are generally incompatible.